The invention proposes a method of managing the supply to a discharge lamp of a motor vehicle headlight.
The invention also proposes a device for managing the supply to a discharge lamp of a motor vehicle headlight.
A discharge lamp consists in particular of two electrodes. The emission of a light flux is due to the creation of an electric arc of light between the two electrodes. To do this, it is necessary to create a significant potential difference between the two electrodes which are arranged in a hermetic bulb, and which are separated from one another by a predetermined distance which may, for example, be 4 mm.
The bulb is generally made of glass, particularly quartz glass, and is filled with a gas such as xenon at a high pressure, with metal salts and with halides in the solid state.
The electric arc struck in the xenon by the action of the switch-on command, supplies the energy needed to evaporate the metal salts and the halides, thus giving rise to the emission of a light flux.
The discharge lamp is supplied through a supply device also known as ballast which activates the lamp and thereafter allows it to operate in a stabilized state at an operating point that is determined by the characteristics of the lamp and the service that it is to provide.
Activation of the lamp may be broken down into a striking phase and a phase of starting-up or warming-up the lamp during which the light flux emitted by the lamp increases until it reaches the predetermined operating point of the lamp.
The striking phase is very short. It allows the electric arc to form between the two electrodes.
During the start-up phase, as is known, the lamp is supplied with a first predetermined current which, in a predetermined length of time, allows the light flux emitted by the lamp to reach the predetermined light flux which corresponds to the operating point of the lamp.
The start-up phase allows the electrodes, and other components of the lamp, such as the gas contained in the bulb, to heat up and reach their operating temperature, allowing the lamp optimum efficiency and optimum light flux.
For example, for the light flux of the lamp to reach 80% of the predetermined flux in two seconds, the lamp is supplied with a supply current of 2.6 amperes.
That makes it possible, when the user switches on his vehicle headlights, for the light flux provided by these quickly to be enough to satisfactorily illuminate the road.
Thereafter, when the lamp is operating in a stabilized state, the supply current to the lamp is reduced to a value of the order of 0.4 ampere.
However, the first predetermined current imposed during the start-up phase causes accelerated ageing of the lamp and this is associated with the evaporation of the electrodes and with the mechanical stresses on the components of the lamp, these being due in particular to the sharp variation in temperature.
In consequence, the life of the lamp is greatly reduced. Indeed, a discharge lamp equipping a vehicle, the headlights of which are switched on only by night, has a life of about 3000 hours, whereas the same bulb can achieve a longer life when the switch-on frequency is lower, for example when the headlights are switched on as soon as the vehicle is started, that is to say even by day.
In order to minimize the ageing of the electrodes, the invention proposes a method of managing the supply to a discharge lamp of a motor vehicle headlight, characterized in that, during the phase of starting up the discharge lamp, the strength of the current supplied to the lamp is controlled on the basis of a parameter representative of the environment of the vehicle so as to optimize the life of the discharge lamp.
According to other features of the invention:
the strength of the current and/or the power supplied to the lamp is limited to a value which is predetermined on the basis of the brightness outside the vehicle;
the parameter is representative of the future change in brightness outside the vehicle.
The invention also proposes a device for managing the supply to a discharge lamp of a motor vehicle headlight, which comprises in particular a device for supplying the discharge lamp which controls the strength of the current supplied to the discharge lamp according to the method according to the process of the type described hereinabove, characterized in that it comprises a device for measuring the brightness outside the vehicle.
According to yet other features of the invention:
the measurement device comprises an external-brightness sensor;
the brightness sensor is designed to be arranged on the inside of a transparent wall, such as the windscreen of the vehicle or the headlight glass;
the measurement device comprises a mapping system which provides the parameter representative of the vehicle environment on the basis of the location of the vehicle with respect to predetermined places such as a tunnel